Clad optical conduit and method of manufacture

ABSTRACT

A flexible, clad, monofilament optical conduit having a flexible light transmitting polymeric core having a relatively high refractive index which is surrounded by a shrunk, heat shrinkable cladding having a relatively low refractive index in comparison to that of the core and which has a relatively small, uniform gap between the outer periphery of the core and the inner periphery of the heat shrinkable cladding, and a method of manufacture whereby a plurality of such conduits are simultaneously produced by polymerization of the conduit core forming material within a length of unshrunken heat shrinkable material which in turn is contained within a relatively thick, durable protective tubing and removal of the relatively thick protective tubing subsequent to polymerization and then shrinking the heat shrinkable material to form a relatively uniform gap between the core and the clad.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to improved clad optical conduit and amethod of production therefor. The improved optical conduit and methodof manufacture of the present invention concern cylindrical lightemitting conduits which emit light from their outer circumferentialsurface in a more optically efficient manner, with a greater lightintensity and in a more uniform manner than previously known. Opticalefficiency refers to the ratio of light output to light input for agiven tube length and cross section. The conduit of the presentinvention includes clad optical conduits in which a glass or polymericcore having a relatively high refractive index is surrounded by acladding of a relatively low refractive index and a gap therebetween iscreated and is filled with air or some other material having arelatively low refractive index compared to that of the core.

Clad optical conduit of the general type within the field of the presentinvention, and their manufacture, are known, and are generallydisclosed, for example, in U.S. Pat. No. 3,641,332 and Application Ser.No. 300,202, filed Jan. 23, 1989, for "Method, Apparatus and Compositionof Matter For a High Temperature Plastic Light Conduit". A copy of Ser.No. 300,202 is filed herewith, to provide basis for subsequentincorporation by amendment. Although such clad optical conduit has beenknown for many years, a number of problems remain even with the improvedconduit as described in application Ser. No. 300,202. Such remainingknown problems include relatively poor optical efficiency, intensity andpoor uniform light emittance at the outer peripheral surface of thecladding which surrounds the inner light transmitting core.

OBJECTS OF THE INVENTION

It is an object of the present invention to provide a clad opticalconduit having improved optical efficiency.

It is a further object of the present invention to provide a clad,optical conduit having greater light emission from its circumferentialperiphery for a given light source.

It is a further object of the present invention to provide a clad,optical conduit having relatively uniform light emission distributionfrom its circumferential periphery even at regions where bends exists.

It is a further object of the present invention to provide a process formanufacture of clad optical conduit having improved optical efficiencyproperties in bent sections of conduit, in comparison to the opticalefficiency properties of known clad optical conduit having the samedegree of bending.

It is a further object of the present invention to provide a process formanufacture of a clad, optical conduit having relatively uniform lightemission from its outer circumferential periphery.

It is a further object of the present invention to provide a process forthe simultaneous manufacture of a plurality of clad, optical conduithaving improved optical efficiency.

It is a further object of the present invention to provide a process forthe simultaneous manufacture of a plurality of clad, optical conduitseach having relatively uniform light emission from its outercircumferential periphery.

It is a further object of the present invention to provide improvedoptical conduit having improved optical efficiency and uniform lightemission properties.

It is a further object of the present invention to provide clad, opticalconduit having a relatively uniform gap between an inner core and anouter cladding.

It is a further object of the present invention to provide a heatshrinkable tube as the clad material on a clad, cylindrical opticalconduit.

It is a further object of the present invention to provide a process forthe manufacture of clad, optical conduit having a heat shrinkablecladding material and a uniform gap between the inside periphery of acladding tube and the outside periphery of an inner core.

SUMMARY OF THE INVENTION

The objects of the present invention are accomplished by the manufactureof a clad, optical conduit in which the conduit or core is surrounded bya shrunk, heat shrinkable tube, such as Teflon™ to provide a snug,uniform cladding around the core, and a relatively thin, uniform gapbetween the cladding and the core. The improved clad, optical conduitmay be made in conventional reactors whereby a conventional monomermixture is placed inside of expanded, heat shrinkable tubing, which inturn has been placed and secured snugly within another tube, or jacketmade of a durable, relatively thick material, progressively polymerizedfrom one end to the other to form a flexible core in a manner so as notto shrink the heat shrinkable tube to its fully shrunk condition priorto when the monomer mixture has achieved the degree of polymerizationdesired for the core while it remains in the reactor; removing theassembly containing jacket, heat shrinkable tubing containing thepolymer core from the reactor; removing the jacket; and applying heat tothe heat shrinkable tubing containing the polymeric core to cause it toshrink and form a snug fit clad around the polymeric core.Alternatively, the core may be made of other optically conductivematerial such as, for example, optical quality glass. The heat shrinkcladding surrounding the core provides a clad, optical conduit havingimproved optical transmission and emission properties both in straightor bent configurations.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view of a section of a conventional clad,optical conduit assembly prior to polymerization.

FIG. 2 is a cross-sectional view of a section of a conventional clad,optical conduit assembly after polymerization.

FIG. 3 is a cross-sectional view of the FIG. 2 clad, optical conduittaken along line 3--3.

FIG. 4 is a cross-sectional view of a section of a clad, optical conduitassembly of the present invention prior to polymerization.

FIG. 5 is a cross-sectional view of a section of a clad, optical conduitassembly of the present invention after polymerization, but prior toapplication of heat.

FIG. 6 is a cross-sectional view of a section of a clad, optical conduitof the present invention after polymerization, after removal of thejacket and after application of heat to shrink the heat shrinkableTeflon™ cladding.

FIG. 7 is a cross-sectional view of the FIG. 6 clad, optical conduittaken along line 7--7.

FIG. 8 is a photograph showing a cross-section of a clad, opticalconduit as disclosed in Ser. No. 883,350, (abandoned) and a clad,optical conduit of the present invention.

FIG. 9 is a photograph showing a light transmitting and emitting bentsection of a clad, optical conduit as disclosed in Ser. No. 883,350,(abandoned) and a light transmitting and emitting a bent section of aclad, optical conduit of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The objects of the present invention are accomplished by a process inwhich clad, optical conduit is manufactured with the improved conduithaving a light transmitting core having a relatively high refractiveindex surrounded by a tubular cladding made of a heat shrinkablematerial having a relatively low refractive index compared to that ofthe core and having a relatively narrow, uniform gap of air or othermaterial between the inner periphery of the cladding and the outerperiphery of the core. The core may be made of polymeric material asdisclosed in Ser No. 300,202 or may be made of glass, or other knownmaterials used for the manufacture of optical conduit. The presentinvention is directed to such clad, optical conduit, the manufacture ofsingle, clad, optical conduit and the simultaneous manufacture of aplurality of such clad, optical conduit.

By reference to FIGS. 1-9 preferred embodiments of the improved clad,optical conduit and their method of manufacture will be described.

Referring first to FIGS. 1-2, known clad optical conduit and theirmethod of manufacture will be described in order to provide a meaningfulcontext within which problems known in the field of the invention willbe discussed and within which the present invention will be described.As is known and described in application Ser. No 300,202, incorporatedby reference as if fully set forth herein, clad, optical conduit may bemanufactured by a process in which cylindrical Teflon™ tubes are filledwith a conventional monomer mixture, placed in a U-configuration in areactor and then polymerized within and along the length of the tubewithin the reactor to form a flexible, light transmitting and emittingcore having a relatively high refractive index surrounded by acomparatively low refractive index Teflon™ cladding. During and afterpolymerization the inner core typically exhibits shrinkage and, inconjunction with the cladding's lack of adhesive properties, causes agap to be created between its outer periphery and the inner periphery ofthe Teflon™ cladding. As shown in FIG. 1, a before polymerization,cross-sectional view of a conventional, clad and jacketed tube assemblyis shown having a flexible jacket 1 surrounding a conventional Teflon™tube 2. Although the tube assembly may be of virtually any length and isgenerally bent into a U-shape for manufacturing and is up to about 40feet in total length, for purposes of illustrating problems currently ofinterest in the field of the invention, a small section of a nominalstraight tube length of about 20 feet having a top end 4 and a bottomend 5 is shown. The liquid monomer mixture 3 filled Teflon™ tube 2 ofthe type described in Ser. No. 300,202 is then polymerized in aconventional manner, such as by progressive immersion of the tubeassembly, from its lower end 5 to its upper end 4, in a relatively warmbath sufficient to permit relatively slow polymerization as alsodescribed in Ser. No. 883,350.

After polymerization, as shown in FIG. 2, the polymerized, clad, opticalconduit, with the flexible jacket 1 removed is illustrated as having asurrounding Teflon™ cladding 6 with its core 7 being of a polymerized,flexible light transmitting polymer having a relatively high refractiveindex. During the polymerization process the core material typicallyshrinks about 12-15 percent in volume. Because polymerization usuallyslowly takes place in the U-shaped tubes, the radial shrinkage issomewhat minimized and the weight of upper, liquid polymerizing mixtureflows downward, tending to fill the Teflon™ tube 2. Also, because pureTeflon™ lacks any known adhesive properties, it pulls away from the wallof the polymerized core and tends to create a gap 8 between the innerperiphery of the Teflon™ tube 6 and the core 7. As discussed in the '332patent, an air gap or a gap filled with equivalent material is essentialto achievement of good optical properties of the finished clad, opticalconduit. Also, because of the shrinkage, the total length of the core 7will decrease in comparison to the length of the monomer filling 3 inthe pre-polymerized Teflon™ tube 2. Thus, the top end 9 and bottom end10 of the polymerized core 7 are shown at different positions withrespect to the Teflon™ clad top end 4 and bottom end 5 than prior topolymerization to illustrate this shrinkage phenomenon; although itshould be appreciated that the actual shrinkage occurs at either end ofthe U-shaped tubes, both of which are at the "top" as described in Ser.No. 300,202. As shown in FIG. 3, a cross-sectional view taken along line3--3 of FIG. 2, the outer Teflon™ cladding 6, the polymerized core 7 andthe gap 8 are shown. Typically gap 8 contains only air, which has anominal refractive index of 1.0. Typically, gap 8 is not uniform inwidth between the outer periphery of core 7 and cladding tube 6, asillustrated in both FIGS. 2 and 3. The gap 8 may contain some otheroptically conductive material besides air, such as for example siliconeoil, so long as the filler material has a relatively low refractiveindex compared to the refractive index of the core 7, whether the corebe of a polymerized material or of some other material such as glass.

Clad, optical conduit made by conventional processes as described inU.S. Pat. No. 3,641,332 and Ser. No. 300,202 have limitations regardingoptical efficiency as well as scattering problems, such as for examplethat they exhibit numerous bright and dark spots, especially at bends inthe conduit. Such optical conduit typically also have loose air pocketswhich can readily be felt and seen along their length. It is presentlybelieved that at least part of the optical problems are associated withthe relatively large and non-uniform gaps 8, as illustrated in FIGS. 2-3and 8-9 of conventional clad, optical conduit.

Conventional clad, optical conduit typically has a relatively thin,i.e., 0.005 to 0.050 inch thickness Teflon™ cladding with an insidediameter from about 1/8 to 3/4 inch. The Teflon™ cladding is aconventional fluorinated ethylene-propylene copolymer, such as forexample fluorinated ethylene propylene sold as FEP TEFLON™ made byDuPont, which may be simultaneously co-extruded with a thermoplasticjacket such as polyethylene or polyvinyl chloride to improve handling ofthe thin tubing. The jacketed Teflon™ tube is filled with athermosetting organic monomer mixture such as set forth in Ser.No.300,202, which in turn is polymerized progressively along the lengthof the tubing assembly by a conventional process as described in SerialNo. 300,202. Due to the relatively low boiling point of the typicalorganic monomers used for the polymeric core, the polymerizationreaction is preferably performed at relatively low temperatures andunder pressure of up to about 250 psi to help avoid formation of bubblesand voids in the core during polymerization. The outer polyethylene orpolyvinyl chloride jacket surrounding the thin Teflon™ tubing functionsto facilitate ease of handling and also functions to protect the thinTeflon™ cladding in two ways. First, because typically a plurality ofthe tube assemblies are loaded inside the manufacturing reactor forsimultaneous polymerization, the outer polyethylene or polyvinylchloride jackets protect the relatively thin Teflon™ tubing fromcollapsing due to the forces exerted by the weight of upper tubingassemblies positioned above relatively lower tubing assemblies. Second,because the thermosetting monomer mixture contracts duringpolymerization, a relatively high vacuum would be created inside of theTeflon™ tubing in the absence of the jacket. Such a vacuum would tend tocollapse the Teflon™ cladding. However, the presence of the jacket wallscreates a pseudo-adhesive force which is not well understood, but whichprevents the Teflon™ cladding from collapsing. During co-extrusion ofthe outer jacket and the inner thin Teflon™ tubing the inside wall ofthe outer jacket and the outside wall of the Teflon™ tubing come tightlytogether. When the hot polymer melt cools and subsequently contracts, apseudo-adhesion force is also created between the outer periphery of thecladding and the inner periphery of the jacket to thus effectivelycounteract the force due to the vacuum created inside of the cladding.

Also, because of uneven contraction of the thermosetting monomer mixtureduring polymerization, both radially and laterally in relation to thecladding, and because the Teflon™ tubing itself does not have a uniforminner diameter along its length, with radial variations of 0.003 to0.005 inches in a typical 1/8" I.D. section of tubing, the range of gapwidths varies along the length of the tubing. In other words, inreference to FIGS. 2-3, the distance between the core 7 and the cladding6, i.e., the gap 8, will vary along any given length of the coated,optical tube and may also vary radially to produce readily observableair pockets.

The significance of non-uniformities in the gap 8 is that when light isintroduced into one end of a straight light conduit, i.e., the coretube, the varying gap widths cause the appearance of bright and darkspots on the outer periphery of the light emitting conduit. Further,when the light emitting conduit is bent or curved to form shapes, at thelocations where the cladding comes in relatively closer proximity to thecore, i.e., at bends, the light appears to be brighter than at straightregions along the conduit. Thus, in both straight and curved clad,optical conduits, the existence of non-uniform gaps defeats and detractsfrom the objective of emitting uniform light from the clad conduit. Inmany cases these relatively bright and relatively dark spots are readilyobservable to the naked eye.

In general, for coated optical tubes within the field of this invention,light is emitted at the end opposite the light source and through thecircumferential periphery along its length. It is believed that the sizeof air gaps, i.e., the distance between the outer periphery of the coreand the inner periphery of the Teflon™ cladding and the length of thegap determine the intensity of emitted light along the gap. Thus, thepresent invention provides for a more intense emission of light along agap for a given length, gap thickness and light source because itsrelatively smaller gap provides for a shorter distance of travel for thelight emitted from the core to the surrounding cladding. Also, incomparison to the conventional clad optical conduit, a greaterpercentage of the total light is emitted from the circumferentialperiphery than from its non-source end. The non-uniformity of light fromthe circumferential periphery is believed to be determined by thenon-uniformity of the air gap. Thus, the present invention, because ofits narrow, uniform air gap in relation to conventional clad opticalconduits, has not only greater intensity of emitted light but alsoimproved uniformity of emitted light.

For example, the FIGS. 8-9 photographs illustrate this phenomenon. Thelower portion of FIG. 8 shows, without magnification, a cross-section ofa clad, optical conduit made in accordance with Ser No. 300,202. To theleft side of the conduit is a very distinct, crescent shaped gap whichmay be seen between the outer periphery of the core and the innerperiphery of the Teflon™ clad. The upper portion of FIG. 8 shows, alsowithout magnification, a cross-section of a clad, optical conduit of thepresent invention. As may be readily observed there is no visible,uneven gap between the core and the clad. Indeed, the clad is so tightlyformed around the core that no gap is visible, it being too narrow to beseen with the naked eye.

Referring to FIG. 9, a light emitting, conventional clad, opticalconduit of Ser. No. 300,202, and a light emitting, clad conduit of thepresent invention are shown with an identical light source at one end ofeach conduit. The conduit of the present invention is the upper conduit,having the larger radius of curvature of the two. As is readilyobservable, the upper conduit is much brighter, thus illustrating thehigher light emission property, or greater intensity of the presentinvention conduit. Also, the presence of non-uniformities in lightoutput illustrated in the lower conduit, especially at its left side.These non-uniformities of emitted light correspond to air gaps which arereadily observable with the naked eye as illustrated in FIG. 8. It isalso readily observable from the FIG. 9 comparative photograph that theconduit of the present invention emits light relatively uniformly alongits entire, bent length, whereas the conventional conduit has a higherintensity at the bend than along its straight portions.

Further in reference to FIGS. 4-7, the use of a heat shrinkable FEPTEFLON™ or other, equivalent heat shrinkable material in a clad, opticalconduit to aid in making a relatively narrow, uniform gap between thecore and the cladding will be described. It has been discovered that useof a heat shrinkable Teflon™ such as polytetrafluoroethylene (TFE),fluorinated ethylene propylene Copolymer (FEP) or perfluoralkoxy resin(PFA) or other heat shrinkable material which has a low refractive indexwill result in production of clad, optical conduit having a uniform gapbetween the core and the cladding. It has also been discovered that notonly will the gap be relatively uniform, but it will also be ofrelatively small, and therefore, of advantageous thickness along theentire length of the light emitting conduit. The optical efficiency andoptical uniformity of straight as well as bent tubes are unexpectedlyimproved in conduit of the present invention compared to conventionalconduit made with a non-shrinkable Teflon™ cladding.

Methods of production and properties of heat shrinkable tubing are wellknown. Heat shrinkable tubing is normally provided in its expanded,i.e., its un-shrunk or pre-shrunk state. With the brief application ofheat such tubing shrinks and molds itself slowly around the form it wasplaced, even the most intricate and irregular shapes, to form, a snug,tight covering. Sources of heat and methods of heating expanded, heatshrinkable material are known and include, for example, by convection,radiation, or excess heat of reaction. Heat shrink tubings are availablein various shrink ratios, such as for example, 1.3 to 1; 2 to 1; 3 to 1;4 to 1; etc. Usually the shrinkage of the heat shrinkable tubing occursonly radially and the length of the tubing usually remains constant.Conventional heat shrinkable tubings are produced from numerousmaterials such as polyvinyl chloride, polyethylene and other poly(olefins) or fluropolymers. These heat shrinkable tubings are alsoavailable in sizes ranging from approximately 1/32" to 6" in diameter.

The present invention is directed to use of heat shrink tubings aspreviously described, preferably in conjunction with the manufacture offlexible, clad, optical conduit as described in Ser. No. 833,350(abandoned). Production of clad, optical conduit of the presentinvention is presently limited to the size of the heat shrink tubingcommercially available, presently believed to be 1/32" to 6" indiameter. It is within the scope of the present invention to producesmaller or larger diameter clad, optical conduits should smaller orlarger diameter heat shrinkable tubing become commercially available.Also, although improved clad, optical conduits of the present inventioncan be prepared with conventional heat shrinkable materials, it ispresently believed that certain impurities are found in heat shrinkabletubing prepared by conventional methods and that these impurities resultin less than optimum optical properties for the finished clad, opticalconduits of the present invention. It is therefore believed andpreferred that heat shrinkable tubing be manufactured by employing inits manufacturing process an inert gas, such as helium, which willproduce a heat shrinkable tubing having relatively fewer impurities andtherefore will result in a finished, clad, optical conduit of thepresent invention having even further enhanced optical properties.

As shown in FIG. 4, a jacket 9 surrounds an un-shrunk or a pre-shrunk,heat shrinkable cladding 10 which in turn surrounds a liquid monomermixture 11. Referring to FIG. 5, the pre-shrunk, heat shrinkablecladding 10 is shown containing the polymerized core 11, which hasexhibited shrinkages as described with reference to FIG. 2. The jacket 9of FIG. 4 has been removed. It may be observed that a narrow gap 12remains between the core 11 and cladding 10. As shown in FIG. 6, afterheat has been applied to the cladding 10, such as by hot air blower forexample, the heat shrinkable Teflon™ cladding 10 has been shrunk to forman even more narrow, relatively uniform gap 14 between the core 11 andthe shrunk Teflon™ cladding 13. FIG. 7, a cross-sectional view takenalong line 7--7 of FIG. 6 illustrates this narrow, uniform gap 14. Gap14 may be of air, or some other material having a refractive indexrelatively small in comparison to the refractive index of the corematerial 11.

EXAMPLE 1

A section of FEP TEFLON™ heat shrinkable tubing with a nominal diameterof 3/8" and having a 1.6 to 1 shrink ratio, Zeus Industrial, Part No. 3H6HS 0, in the expanded state was filled with a typical, conventionalformulation of a thermosetting monomer mixture and was polymerized underpressure. After polymerization, the outer surface of the sample appearedto be non-uniform upon visual inspection. However when the sample wasexposed briefly to heat, the heat shrinkable tubing exhibited uniform,radial shrinkage and the resulting clad conduit had a very uniformappearance based upon visual inspection. This sample was a laboratoryscale sample in which a single tube was prepared. It appears from thissample that clad conduit having cross-section as shown in FIG. 7 can bemade with substitution of a heat shrinkable thermal plastic cladding inplace of a non-shrinkable type cladding.

Typically, however, for commercial quantities, clad, optical conduitsare batch processed in reactors in which a plurality of tube assembliescontaining monomer mixture are simultaneously polymerized and theabove-described method may not yield good results in such batchprocessing.

EXAMPLE 2

A conventional reactor was loaded with a plurality of heat shrinkabletubing assemblies and filled with a conventional monomer mixture andthen processed in the conventional fashion. It was discovered that thethin, heat shrinkable Teflon™ tubings could not support themselves andtheir walls collapsed at various locations, as was readily observableupon visual inspection and which thus resulted in non-uniform gapsbetween the core and the cladding.

To alleviate this collapsing problem, as referred to previously withrespect to conventional processing, a relatively thick, chemicallystable flexible jacket could be used outside of the heat shrinkcladding. Although it is conceivable with present technology toco-extrude FEP TEFLON™ tubing inside of either a polyethylene orpolyvinyl chloride or other polymer jacket via customary, continuousform melt extrusion, this would not, as a practical matter, be effectivewith heat shrinkable Teflon™ tubing, because the heat released duringthe hot melt extrusion of the jacketing would cause the heat shrinkableTeflon™ tubing to shrink from its expanded state to the shrunken stateand thus result in defeating the main purpose for which the heatshrinkable tubing is used, that is, the creation of a narrow, uniformgap between the tube and the core in the tubing after the polymerizationreaction has occurred. It is important to the present invention that theheat shrink occur during and/or after polymerization, rather than beforepolymerization of the core material so that a narrow, uniform gapbetween the core and the cladding results.

EXAMPLE 3

A four-feet long sample of the same heat shrinkable tubing mentioned inExample 1 was placed inside of a four-feet long section of polyethylenetubing. The polyethylene tubing or jacket was also cut along its entirelength in order to facilitate insertion of the heat shrinkable tubing.The diameter of the tubing was chosen so that the heat shrinkabletubing, in its expanded form, fit very snugly inside of the polyethylenetubing. The thus nested tubings were wrapped with tape so that the heatshrinkable tubing was completely covered. The taped, nested tubings werethen filled with a conventional thermosetting monomer mixture by placingthe monomer inside of the heat shrinkable tubing. The monomer was thenpolymerized under pressure. After completion of polymerization, thepolyethylene jacket was peeled off and the heat shrinkable tubing whichcontained the flexible, polymerized tube was of non-uniform, generallycylindrical configuration along its entire length upon visualinspection. When the polymerized section of Teflon™ cladding was exposedto heat, the heat shrinkable Teflon™ cladding shrank radially andyielded a clad, monofilament conduit which had a narrow, uniform,cylindrical appearance along its entire length upon visual inspection.The clad, monofilament conduit also exhibited fairly uniform lightemission along its entire length and circumferential periphery, both instraight sections and bent sections.

EXAMPLE 4

A clad, monofilament conduit was made by the same procedure as inExample 3, except that a polyvinyl chloride jacket, which was also slitlongitudinally, was used to wrap the heat shrinkable tubing and exceptthat the slit was closed by gluing after insertion of the heatshrinkable tube. Other means to close the jacket may be used such as bytape, laser induced bonding or by other conventional methods. Afterpolymerization and after heat shrinking of the heat shrinkable Teflon™cladding, the clad conduit was of uniform, cylindrical contour along itsentire length and emitted light of a very uniform flux along its entirelength and periphery, both in straight sections and bent sections.

EXAMPLE 5

A plurality of conduit of the type described in Example 4 were preparedand loaded into a conventional polymerization reactor and polymerized,as described in Ser. No. 883,350 (abandoned). When the polyvinylchloride jackets were peeled off the samples, each of the cladmonofilament conduits in an unshrunken condition, had non-uniform,generally cylindrical contours along their entire length. Each of theseconduits was then subjected to heat treatment and after treatment thefinished, clad, conduits had uniform cylindrical contours along each oftheir entire length by visual inspection. Also each conduit was lighttested and the light emitted from each conduit was very uniform alongthe entire length and along the circumference of each conduit uponvisual inspection, both in straight and bent configurations.

EXAMPLE 6

A linear glass rod approximately 3' long with an outside diameter of0.375" was cleaned with soap and water then washed with acetone twiceand completely dried and flamed in an oven. The rod was then insertedinside an FEP heat shrinkable tubing, approximately 0.410" insidediameter with a wall thickness of 0.012". The heat shrinkable tubing wasthen shrunk around the rod with the application of heat. One end of therod was placed in an illuminator and then illuminated. The rod emittedlight out of its outer periphery much like a neon lamp. Change of colorin the rod could be observed by changing the color of light at thesource of light in the illuminator. The glass rod was not of opticalquality, therefore the attenuation of the light was readily observable.It is believed, however, that use of an optical quality glass rod wouldsignificantly reduce attenuation of the light without effect onapplication of a heat shrinkable cladding to the glass rod to form auniform, narrow gap therebetween.

As shown by these examples the heat shrink Teflon™ provides a means toform a narrow, uniform gap surrounding the core of a flexible, clad,monofilament conduit and/or of other optical conduit such as a glassrod.

As may be seen from the above-described examples, an improved, clad,optical conduit and method of manufacture are provided. Numerousmodifications, alterations, alternate embodiments and alternate methodsof manufacture may be contemplated by those skilled in the art and maybe utilized in accomplishing the present invention of providing acladding and a narrow, uniform gap surrounding the core of the opticalconduit. Optical conduit and methods of manufacture as expressly setforth in the description of the preferred embodiments are not intendedto be limitations of the present invention, but rather as illustrationsof the inventive concepts of the present invention. It is envisionedthat all such alternate means for forming a narrow, uniform gapsurrounding the core are within the scope of the present invention asdefined by the present claims.

I claim:
 1. A flexible, clad monofilament conduit comprising:acylindrical, flexible polymeric core having a relatively high refractiveindex; a tubular, flexible, cladding surrounding the core; and means forestablishing a relatively narrow and substantially uniform gapcontaining air between the outer periphery of the core and the innerperiphery of the cladding.
 2. The conduit of claim 1 wherein thecladding is a heat shrinkable fluorocarbon.
 3. The conduit of 1 claimwherein the cladding is made of fluorinated ethylene-propylenecopolymer.
 4. The conduit of claim 1 wherein the cladding is made ofperfluoralkoxy resin.
 5. The conduit of claim 1 wherein the cladding ismade of polytetrafluoroethylene.
 6. An optical conduit comprising:acylindrical, optically conductive core having a relatively highrefractive index; a tubular heat shrinkable cladding having a relativelylow refractive index and in a shrunk condition surrounding the core andproviding a relatively narrow, substantially uniform gap between theouter periphery of the core and the inner periphery of the cladding. 7.The process of making a flexible, clad monofilament conduit comprisingthe steps of:selecting a length of relatively durable polymeric tubularjacket; cutting a slit along the length of the jacket; selecting alength of tubular cladding made of un-shrunken heat shrinkablefluoropolymer tubing; inserting a length of the tubular cladding insidethe jacket; placing a polymerizable monomer mixture inside the tubularcladding; polymerizing the mixture to form a flexible tubular core;removing the jacket; and applying heat to the cladding effective toradially shrink the cladding to form a relatively narrow, uniform gapbetween the outer periphery of the core and the inner periphery of thecladding.
 8. A flexible, clad monofilament conduit comprising:acylindrical, flexible polymeric core having a relatively high refractiveindex; a tubular, flexible, un-shrunken heat shrinkable claddingsurrounding the core; and a tubular, thermoplastic jacket made of apolymeric material.
 9. The product made by the process of:selecting atubular, flexible, un-shrunken, heat shrinkable material to be used toform a cladding; selecting a monomer mixture; placing the mixture insideof the cladding; polymerizing the mixture to form a solid, flexible,generally cylindrically shaped core surrounded by the un-shrunken, heatshrinkable cladding to form an un-shrunk conduit; and heating theun-shrunk conduit at a relatively high temperature and for a timesufficient to shrink the heat-shrinkable cladding about the core tominimize a gap between the core and the cladding.
 10. The product ofclaim 9 further including the step of placing the tubular, flexible,un-shrunken, heat shrinkable cladding inside of a tubular polymericjacket material, said step being performed prior to the step of placingthe monomer mixture inside of the cladding.
 11. The product of claim 9wherein the tubular, flexible, un-shrunken, heat shrinkable material isa fluoropolymer.